1. Field of the Invention
The present invention relates to a solid polymer electrolyte-catalyst composite electrode, an electrode for a fuel cell and a process for producing these electrodes.
2. Description of the Related Art
As an electrochemical apparatus having an ion-exchange membrane as a solid electrolyte, for example, there are a solid polymer type water electrolysis cell and a solid polymer type fuel cell.
The solid polymer electrolyte type water electrolysis cell is an apparatus having an ion-exchange membrane, for example, such as perfluorosulfonic acid membrane as an electrolyte and an anode and a cathode connected to the respective side of the ion-exchange membrane which supplies oxygen from the anode and hydrogen from the cathode when D.C. voltage is applied across the both electrode while the anode is being supplied with water.
The electrochemical reactions which take place on the two electrodes will be described below.
Anode: H2Oxe2x86x921/2O2+2H++2exe2x88x92
Cathode: 2H++2exe2x88x92xe2x86x92H2 
Total reaction: H2Oxe2x86x92H2+1/2O2 
It can be seen in these reaction formulae that the anode reaction proceeds only on a three-phase interface which allows the reception of water as an active material and the delivery of oxygen as a product, proton (H+) and electron (exe2x88x92) at the same time while the cathode reaction proceeds only on a three-phase interface which allows the reception of proton (H+) and electron (exe2x88x92) and the delivery of hydrogen at the same time.
On the other hand, the solid polymer electrolyte type fuel cell is an apparatus having an ion-exchange membrane, for example, such as perfluorosulfonic acid membrane as an electrolyte and an anode and a cathode connected to the respective side of the ion-exchange membrane which generates electricity due to electrochemical reaction developed by the supply of hydrogen to the anode and oxygen to the cathode.
The electrochemical reactions which take place on the two electrodes will be described below.
Anode: H2xe2x86x922H++2exe2x88x92
Cathode: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
Total reaction: H2+1/2O2xe2x86x92H2O
It can be seen in these reaction formulae that the both electrode reactions proceed only on a three-phase interface which allows the reception of gas (hydrogen or oxygen) and the delivery or reception of proton (H+) and electron (exe2x88x92) at the same time.
An example of the electrode, used in the apparatus, having such a function is a solid polymer electrolyte-catalyst composite electrode comprising a solid polymer electrolyte and catalyst particles. The structure of this electrode with a fuel cell as an example will be explained.
FIG. 12 is an explanation view showing the structure of this electrode. This electrode is a porous electrode comprising catalyst particles 121 and a solid polymer electrolyte 122 three-dimensionally distributed in admixture and having a plurality of pores 123 formed thereinside. The catalyst particles form an electron-conductive channel, the solid electrolyte forms a proton-conductive channel, and the pore forms a channel for the supply and discharge of oxygen, hydrogen or water as product. The three channels are three-dimensionally distributed and numerous three-phase interfaces which allow the reception or delivery of gas, proton (H+) and electron (exe2x88x92) at the same time are formed in the electrode, providing sites for electrode reaction. Incidentally, reference numeral 124 represents an ion-exchange membrane.
The preparation of an electrode having such a structure has heretofore been accomplished by the following process. There is a process which comprises applying a paste made of catalyst particles and a solution having PTFE particles (polytetrafluoro ethylene) dispersed therein to a polymer film or a carbon electrode substrate of an electro-conductive porous material to make a film (normally having a thickness of from 3 to 30 xcexcm), heating and drying the film, and then applying a solid polymer electrolyte solution to the film so that the film is impregnated with the solution. Alternatively, there is a process which comprises applying a paste made of catalyst particles thereon, PTFE particles and a solid polymer electrolyte solution to a polymer film or a carbon electrode substrate of an electro-conductive porous material to make a film (normally having a thickness of from 3 to 30 xcexcm), and then heating and drying the film. As the solid polymer electrolyte solution, there is used a solution obtained by dissolving the same composition as the aforementioned ion-exchange membrane in an alcohol. As the solution having PTFE particles dispersed therein, there is used a solution having PTFE particles having a particle diameter of about 0.23 xcexcm dispersed therein.
The solid polymer electrolyte-catalyst composite electrode comprising metal particles of the platinum group or oxide particles of metal of the platinum group as a catalyst is used in a water electrolysis cell or a fuel cell. On the other hand, the solid polymer electrolyte-catalyst composite electrode comprising platinum group metal supported on carbon as a catalyst is used in a fuel cell.
The aforementioned solid polymer electrolyte-catalyst composite electrode has the following two disadvantages.
One of the two disadvantages is that the solid polymer electrolyte-catalyst composite electrode has a high resistivity. The reason of this disadvantage is as follows.
When catalyst particles are mixed with solid polymer electrolyte solution to prepare a paste. The catalyst particles are covered with solid polymer electrolyte film having no electronic conduction and a pore (void) 132 and a solid polymer electrolyte 133 exist between catalyst particles 131 even after film-making process to prepare an electrode. The formation of a continuous catalyst particle passage (electro-conductive channel) is inhibited, though forming a continuous solid electrolyte passage (proton-conductive channel), as shown in the sectional view of electrode of FIG. 13.
The other disadvantage is that if a solid polymer electrolyte-catalyst composite electrode comprising the platinum group metal supported on carbon as a catalyst is used in an electrode for a fuel cell. The resulting percent utilization of catalyst supported on carbon is as low as about 10% as reported in Edson A. Tichianlli, xe2x80x9cJ. Electroanal. Chem.xe2x80x9d, 251, 275 (1998).
This is caused by the fact that the preparation process of supporting catalyst such as platinum on carbon particle, and then mixing the carbon particle with a solid polymer electrolyte.
In other words, the carbon particles as a support has a particle diameter as small as 30 nm. Thus, the carbon particle to be mixed with the solid polymer electrolyte has an aggregation of a few carbon particles that gives a carbon particle aggregate having a dense unevenness formed on the surface thereof. On the other hand, the solid electrolyte solution is viscous. Thus, regardless of which is used the process which comprises impregnating the layer having carbon particles and PTFE particles dispersed therein with a solid polymer electrolyte solution or the process which comprises the use of a paste obtained by mixing carbon particles, PTFE particles and a solid polymer electrolyte solution, the solid polymer electrolyte solution cannot penetrate deep into the central portion of the carbon particle aggregate. As a result, it is impossible to form a three-phase interface in the deep portion of the carbon particle aggregate. Accordingly, the catalyst particles disposed in the deep portion of the carbon particle aggregate does not take part in the electrode reaction to thereby cause decrease of percent utilization of catalyst.
The structure of such an electrode is shown in FIG. 14. As shown in FIG. 14, carbon particles 143 having catalyst particles 141, 142 supported thereon are aggregated to form a carbon particle aggregate (four of the carbon particles are shown forming the aggregate). Thus, the solid polymer electrolyte 144 can not penetrate into the central portion 145 of the indented portion. Accordingly, a catalyst particle 141 which is disposed at the surface of carbon in contact with the solid polymer electrolyte to effectively contribute to the electrode reaction and a catalyst particle 142 which does not come in contact with the solid polymer electrolyte and thus can not effectively contribute to the electrode reaction are formed in the catalyst particles.
It is an object of the present invention to decrease resistance of the electrode by adding the electro-conductive passage (electro-conductive channel) to the solid polymer electrolyte which has the proton-conductive passage inherently.
It is another object of the present invention is to improve the percent utilization of catalyst in the electrode for a fuel cell by improving the structure of microscopic three-phase boundary of the electrode.
A porous solid polymer electrolyte-catalyst composite electrode according to the present invention comprises: a solid polymer electrolyte; catalyst particles; an electro-conductive material which is supported on a proton-conductive region in the solid polymer electrolyte.
An electrode for a fuel cell, comprises a solid polymer electrolyte-catalyst composite electrode containing a solid polymer electrolyte, carbon particles and a catalyst material; wherein the catalyst material is supported mainly on the surface of the carbon particles in contact with a proton-conductive passage in the solid polymer electrolyte.
A process for the preparation of a solid polymer electrolyte-catalyst composite electrode, comprises the steps of: preparing a porous solid polymer electrolyte-catalyst composite electrode parent body comprising a solid polymer electrolyte and a catalyst particles; adsorbing a starting catalyst material compound into the solid polymer electrolyte in the electrode parent body; and reducing the starting catalyst material compound so that the catalyst material is deposited into the solid polymer electrolyte in the electrode.
A process for the preparation of an electrode for a fuel cell, comprises the steps of: a first step, adsorbing a starting catalyst material compound into the solid polymer electrolyte in a mixture including solid polymer electrolyte and carbon particle; and a second step, reducing the starting material catalyst material compound by chemical reduction. In the process, the operation of the first and second steps is repeated twice or more times.